The use of cerium fluoride as a scintillator material in the detection of ionizing radiation is disclosed in application Ser. No. 07/218,234, filed July 12, 1988, and incorporated herein by reference. There, the use of cerium fluoride as a scintillator material in gamma ray detectors for positron emission tomography ("PET") is also disclosed.
The value of PET as a clinical imaging technique is in large measure dependent upon the performance of the detectors. The typical PET camera comprises an array of detectors consisting of scintillator crystals coupled to photomultiplier tubes (PMTs). When a high energy photon or gamma ray strikes a detector, it produces light in the scintillator crystal that is then sensed by the PMT, which registers the event by passing an electronic signal to the reconstruction circuitry. The scintillator crystals themselves must have certain properties, among which are (1) good stopping power, (2) high light yield, and (3) fast decay time.
As applied to scintillators, stopping power is the ability to stop the 511 keV photons associated with PET in as little material as possible so as to reduce the overall size of the detector, of which the scintillator crystals form a substantial portion. Stopping power is typically expressed as the linear attenuation coefficient (tau) having units of inverse centimeters (cm.sup.-1). After a photon beam has traveled a distance "x" in a crystal, the proportion of photons that have not been stopped by the crystal is calculated as follows: EQU fraction of unstopped photons=e.sup.(-tau * x).
Thus, after traveling a distance of 1/tau (the "absorption length"), approximately 37% of the photons will not have been stopped; 63% will have been stopped. Likewise, 63% of the remaining photons will have been stopped after traveling an additional distance of 1/tau. For PET and other applications involving the detection of ionizing radiation, it is desirable for 1/tau to be as small as possible so that the detector is as compact as possible.
Light yield is also an important property of scintillators. Light yield is sometimes referred to as light output or relative scintillation output, and is typically expressed as the percentage of light output from a crystal exposed to a 511 keV photon beam relative to the light output from a crystal of thallium-doped sodium iodide, NaI(Tl). Accordingly, the light yield for NaI(Tl) is defined as 100.
A third important property of scintillators is decay time. Scintillation decay time, sometimes referred to as the time constant or decay constant, is a measure of the duration of the light pulse emitted by a scintillator, and is typically expressed in units of nanoseconds (nsec). As discussed in application Ser. No. 07/218,234, if a scintillator's decay constant is short, then more of its time will be available for the detection of ionizing radiation, for example, in the case of PET, coincident photons, and the scintillator can be employed in high rate applications.
In addition to the three important properties discussed above, scintillator crystals should be easy to handle. For example certain known scintillators are hygroscopic, i.e., they retain moisture, making it necessary to very tightly encapsulate them to allow their use in detectors. These hygroscopic scintillators are more difficult to use.
Prior to cerium fluoride (CeF.sub.3), known scintillators included (1) plastic (organic) scintillators, (2) gadolinium orthosilicate (Gd.sub.2 SiO.sub.5, also referred to as "GSO"), (3) thallium-doped sodium iodide (NaI(Tl)), (4) undoped cesium iodide (CsI) and thallium-doped cesium iodide (CsI(Tl)), (5) cesium fluoride (CsF), (6) bismuth germanate (Bi.sub.4 Ge.sub.3 O.sub.12, also referred to as "BGO"), and (7) barium fluoride (BaF.sub.2).
Plastic (organic) scintillators, typically composed of polystyrene doped with a wavelength-shifting additive, are commercially available under such tradenames as PILOT U and NE 111. Upon excitation with a 511 keV photon, plastic scintillators emit a light pulse having a very fast decay constant of approximately 1.5 nsec and light output proportional to the energy of the incident photon. The main disadvantage of plastic scintillators is their low density (approximately 1.1 to 1.2 g/cm.sup.3) due to the light atoms (hydrogen and carbon) that make up the molecules of the material. Because of their low density, plastic scintillators have poor stopping power, and are therefore poorly suited for use in PET and other applications involving the detection of ionizing radiation.
GSO, gadolinium orthosilicate (Gd.sub.2 SiO.sub.5) is a scintillator well suited for PET with good stopping power, high light yield, and reasonable decay constant. The disadvantage of GSO is that it is very difficult to manufacture and prohibitively expensive, costing about fifty times as much as BaF.sub.2 and CeF.sub.3, and twenty times as expensive as BGO.
NaI(Tl), thallium-doped sodium iodide, has the best light output of the prior known scintillators listed above. NaI(Tl) also has reasonably good stopping power (1/tau=3.0 cm at 511 keV). However, NaI(Tl) has a long decay constant (250 nsec), a significant disadvantage for use in PET and other time-of-flight applications. NaI(Tl) is also highly hygroscopic, making it extremely difficult to handle in that it must be encapsulated in bulky cans.
CsI(Tl), thallium-doped cesium iodide, is not particularly well suited for PET because its decay time is greater than 1000 nsec, far too long for high rate applications like PET. Undoped CsI appears suitable for PET, although its slow component is a disadvantage for high rates. This slow component can be fairly effectively removed electronically, however.
CsF, cesium fluoride, has been used successfully in PET. CsF has two main disadvantages: first, it has a rather poor stopping power (absorption length (1/tau)=2.3 cm at 511 keV) and second, it is extremely hygroscopic. The poor stopping power of CsF limits its ability to localize the origin of the gamma rays in PET. The hygroscopic nature of CsF makes it difficult to handle.
BGO has the highest density (7.13 g/cm.sup.3) of the prior known scintillator materials listed above. Its stopping power is the best (1/tau=1.1 cm at 511 keV) and, as a result, BGO is best able to absorb 511 keV photons efficiently in small crystals. However, BGO's very long decay constant (300 nsec), longer even than NaI(Tl), is a significant disadvantage for use in PET and other high rate applications involving the detection of ionizing radiation.
The use of BaF.sub.2 as a scintillator material is described in Allemand et al. U.S. Pat. No. 4,510,394. BaF.sub.2 emits light having two components: a slow component having a decay constant of approximately 620 nsec and a fast component having a decay constant of approximately 0.6 nsec. BaF.sub.2 has a light yield of approximately 16% that of NaI(Tl) and about half the stopping power of BGO (1/tau=2.3 cm at 511 keV). Unlike CsF and NaI(Tl), BaF.sub.2 is not hygroscopic.
The fast component of BaF.sub.2 emits light in the ultraviolet region of the spectrum. Glass photomultiplier tubes are not transparent to ultraviolet light, so a quartz photomultiplier tube must be used to detect the fast component of BaF.sub.2. Since quartz photomultiplier tubes are substantially more expensive than glass (by a factor of two), one would prefer to avoid using BaF.sub.2, if possible, in favor of using a scintillator that can be detected by a glass photomultiplier tube. The fast component gives BaF.sub.2 very good timing resolution, but the slow component limits its high rate capabilities. In other words, it takes BaF.sub.2 longer to get ready for the next event. (This slow component can be fairly effectively removed electronically, however).
Of the prior known scintillator materials, BGO has the best stopping power, NaI(Tl) has the best light yield, and BaF.sub.2 has the best timing resolution. However, as noted above, some of these known materials have significant shortcomings which hinder their performance as scintillators for PET and other applications involving the detection of ionizing radiation: BGO has a very long decay constant; NaI(Tl) also has a very long decay constant and is hygroscopic. Of these materials, BaF.sub.2 has the best of stopping power, light output and decay constant, and does not present a problem with hygroscopy. However, the slow component of BaF.sub.2 does limit its rate capabilities.
As disclosed in application Ser. No. 07/218,234, cerium fluoride, CeF.sub.3, has been found to provide a balance of stopping power, light yield and decay constant that is superior to previously known scintillator materials. As a result, cerium fluoride is favorably suited for use as a scintillator in positron emission tomography and other applications involving the detection of ionizing radiation. The relevant properties of CeF3 as compared to those of prior known scintillator materials, are shown in FIG. 1.
As shown in FIG. 1, CeF.sub.3 provides a balance of stopping power, light yield and decay constant that is superior to other known scintillator materials. In particular, CeF.sub.3 exhibits a fast component of approximately 5 nsec and a slow component having a decay constant of approximately 30 nsec, both far superior to those of NaI(Tl) and BGO. With respect to light yield, CeF.sub.3 exhibits a value of 4-5 percent that of NaI(Tl); its light yield is thus about one-half that of BGO. In addition, the absorption length (stopping power) of CeF.sub.3 (1/tau=1.9 cm at 511 keV) is between that of BGO and BaF.sub.2. Finally, CeF.sub.3 exhibits no hygroscopy. In contrast to NaI(Tl), CeF.sub.3 is superior in that its decay constant is far shorter and it is not hygroscopic, making it much easier to handle than NaI(Tl).
As further shown in FIG. 1, in contrast to BaF.sub.2, CeF.sub.3 has superior stopping power (1/tau) but inferior light yield. In addition, CeF.sub.3 has a fast component like BaF.sub.2. Moreover, while the fast component of BaF.sub.2 can only be detected using expensive quartz photomultiplier tubes, the fast component of CeF.sub.3 can be detected using much less expensive glass photomultiplier tubes. Finally, as shown in FIG. 1, in contrast to BGO, CeF.sub.3 has inferior absorption length (stopping power) and light yield, but has a decay constant far superior to that of BGO. Thus, CeF.sub.3 provides adequate stopping power and light yield with an improved decay constant.
In developing a commercial grade CeF.sub.3 scintillator, the main objectives are to produce high quality CeF.sub.3 scintillators reliably and at low cost. If possible, it is desirable to produce large crystals of uniformly clear, scatter-free CeF.sub.3, while avoiding the use of extremely pure and thus expensive CeF.sub.3 as a starting material. The use of extremely pure CeF.sub.3 is prohibitively expensive on a commercial scale because of the difficulty in purifying CeF.sub.3 in large quantities. Thus, if lower purity CeF.sub.3 could be employed as a starting material, there would be a substantial reduction in the cost of producing cerium fluoride scintillators on a commercial scale.